This invention is related to a process and apparatus for processing a metal, such as a low carbon steel, to obtain a predictable amount of a particular micro-structure such as Bainite, or to obtain a predictable tensile strength, or a predictable reduction in cross-section. The specimen is elongated as it is being heated above the critical temperature for creating Austenite. The temperature of the specimen is then lowered in a molten salt bath to a temperature plateau corresponding to the Bainite critical temperature, for a predetermined period of time so that the percentage of Bainite, the ultimate tensile strength of the steel and other factors can be predicted from several specimens, and then repeated in a commercial process.
Thermastress process is a thermo-mechanical process developed over the past few years for producing steel and steel alloys with remarkable physical characteristics. The process differs from other conventionally used methods for making steel by deforming the steel material simultaneously with a rapid cooling step. Whereas high strength steels produced by processes based on U.S. Pat. No. 3,378,360, which issued to William H. McFarland on Apr. 16, 1968, are limited to relatively thin sections, in order to achieve the high rate of temperature drop to attain essentially a Martensitic micro-structure, my Thermastress process is capable of producing sections greater than 0.375 inches. This is because the transformation of austenitized steel is accomplished by an apparent shift of the critical temperature for producing Bainite (B.sub.s) brought about by the simultaneous application of stress and plastic deformation imposed on the steel. Further, the process inherently tends to produce Bainite rather than Martensite.
My early process was disclosed in U.S. Pat. No. 3,964,938 which issued June 22, 1976 for a "Method And Apparatus for Forming High Tensile Steel from Low and Medium Carbon Steel".
The basic Thermastress process involves moving material between two spaced driving means immediately adjacent heating and quenching zones. The effect of the two zones is to impose a temperature gradient on the material between the two drives so that after a gradual temperature rise, for example, to around 2,000.degree. F., a rapid temperature drop is imposed on the processed material.
The relative speeds of the upstream drive and the downstream drive are so controlled that the ratio of the two drives can be changed without affecting the value of the material input speed.
If the ratio between the downstream drive with respect to the upstream drive exceeds unity, the processed material is stretched as it passes through the heating zone, where the yield strength of the material is substantially lowered. A condition of dynamic equilibrium occurs between the two drives as the material accelerates toward the downstream drive, establishing a very stable cross-section reduction profile with the cross-section of the processed material being reduced in inverse proportion to the increase in velocity. The final cross-section of the material obtained by elongation remains constant within very close dimensional tolerances.
In the case of low and medium carbon steel, the effect of a simultaneous rapid temperature drop as the material passes from the heating zone into the quenching zone, in conjunction with the plastic flow taking place, is to substantially modify the steel micro-structure. The fine grained micro-structure, thus produced, brings about an increase in the ultimate tensile strength as high as 220,000 p.s.i. and above at diameters, exceeding by a factor greater than 10, the thickness of high strength steel produced by the rapid quenching of conventional heated-finished sheet steel. Steel produced by the Thermastress process possesses excellent welding properties due to its relatively low carbon content.
One phenomenom related to the commercial Thermastress process is that the critical temperature, at which the micro-structure of steel neucleates to Bainite, as its temperature is being reduced, shifts upwardly, compared to the conventional time temperature curves for the micro-structure of such steels.
Heretofore, the process for forming Bainite has been either to increase the temperature of the steel to a temperature above the critical temperature to form Austenite, and then to lower the specimen to the critical B.sub.s temperature, missing the TTT nose, and maintaining the temperature stable at the B.sub.s temperature, for a time sufficient for the micro-structure of the steel to change to Bainite, a period that can take hours.
In the Thermastress process, the period for the micro-structure change to occur is significantly reduced. It is also believed that the elongation process causes the critical temperature for the formation of Bainite (B.sub.s) as well as to shift upwardly in a pattern believed to be unknown to those skilled in the art. Further, even though the cooling curve in the Thermastress process does not miss the TTT noze, a substantial amount of Bainite is formed. That is to say there are no time vs. temperature, published curves for the formation for Martensite or Bainite as formed by the Thermastress process.
One approach for determining such a curve for a particular steel is to raise the temperature of a number of specimens to form Austenite and then to reduce the temperature of each specimen to a plateau at the critical temperature for Bainite (B.sub.s). A set of specimens made at different periods along the temperature plateau and in a range of plateaus, and analyzed for Bainite content, will make the concentration of Bainite predictable for different steels and enable establishing the shift in B.sub.s level as related to the degree of material elongation.